Aflatoxin B1 is one of the most potent chemical carcinogens found in the human environment. This and other aflatoxins are common contaminants in agricultural products. Our previous studies have shown that aflatoxin B1 and G1 can be converted by hamster, rat or mouse liver homogenates to metabolites which are mutagenic in Neurospora, and that aflatoxin B1 is much more mutagenic than aflatoxin G1 in an in vitro activation system using liver or kidney homogenates from rats or hamsters. Further studies on the relationship between carcinogenesis and mutagenesis and the mechanism of mutagenesis show that aflatoxin B1 per se appears to be a weak mutagen in the resting conidia of the ultraviolet-repair deficient strain UVS-2 of Neurospora. It seems that aflatoxin B1 causes certain genetic damage which can be repaired by the wild type but not by the UV-sensitive strain.